Class 12 Biology Chapter 3 Human Reproduction Notes

Class 12 Biology Chapter 3 Human Reproduction Notes: Access comprehensive notes and solutions for Class 12 Biology Chapter 3 on Human Reproduction. Dive into the intricacies of human reproductive systems, fertility, contraception, and more with detailed explanations, diagrams, and key concepts to enhance your understanding.

Class 12 Biology Chapter 3 Human Reproduction Notes

Human Reproductive System:

The Human Reproductive System mainly consists of:

The Male Reproductive System:

The male reproductive system is positioned in the pelvis region and comprises a pair of testes in addition to the accessory glands, ducts, and the external genitalia. A pouch-like structure known as scrotum encloses the testes located outside the abdominal cavity Each testis has close to 250 testicular lobules(compartments). These lobules comprise 1-3 seminiferous tubules wherein the sperms are produced. the lining of these tubules consists of two types of cells – male germ cells and sertoli cells The exterior of these tubules consist of spaces containing blood vessels and Leydig cells Male sex accessory ducts comprises rete testis, vasa effrentia, epididymis and vas deferens The urethra opens externally to the urethral meatus The male external genitalia, the penis is covered by foreskin which is a loose fold of skin.

The Female Reproductive System:

The female reproductive system is made up of the internal and external sex organs, which consists of a pair of ovaries and oviducts, cervix, uterus, vagina and the external genitalia situated in the pelvic region. Along with the mammary glands, these female reproductive organs are combined both structurally and functionally in order to support the complete processes of reproduction including ovulation, fertilization, pregnancy, and the birth of a child. The female accessory ducts are constituted by the oviducts, vagina and uterus The section closer to the ovary is funnel-shaped infundibulum that possesses the fimbriae – finger-like projections facilitating the assimilation of ovum post ovulation The infundibulum directs to a wider section of oviduct known as ampulla. The last section of the oviduct, isthmus, has a narrow lumen joining the uterus. Uterus is also known as the womb The cervical cavity is known as the cervical canal which goes onto form the birth canal along with the vagina Female external genitalia comprises – mons pubis, labia minora, labia majora, clitoris and hymen Both the male and female reproductive systems play an important role in the process of reproduction. Other than these reproductive organs, there are sex hormones which are produced by the respective glands and are mainly involved in the development of secondary sexual characteristics and proper functioning of the reproductive tracts.

Gametogenesis:

The process of formation of male and female gametes in testes and ovary respectively is called gametogenesis.

It is of two types:

• Spermatogenesis in males
• Oogenesis in females

Spermatogenesis:

In testes immature, male germ cells (spermatogonia) produce sperm by spermatogenesis that begin at puberty.

The spermatogonia present at the inner side of seminiferous tubules multiply by mitotic division and increase in number. Each spematogonium contain 46 chromosomes.

Spermatogonia forms spermatocyte that undergo meiotic division to reproduce secondary spermatocytes having 23 chromosomes.

The spermatids are transformed into spermatozoa by the process called spermiogenesis. The sperm heads remain embedded in sertoli cells and are released from seminiferous tubules by the process of spermiation.

Hormonal control of spermatogenesis:

Spermatogenesis initiated due to increase in secretion of gonadotropin releasing hormone by hypothalamus.

Increase in GnRH act on anterior pituitary and stimulate secretion of two gonadotropins, LH and FSH.

LH acts on Leydig cells and stimulates them to secrete androgens.

FSH acts on Sertoli cells, stimulates secretion of some factors which help in spermiogenesis.

Structure of sperm:

Sperm is a microscopic structure composed of a head, neck, a middle piece and a tail. The sperm head contain elongated haploid nucleus, anterior portion of which is covered by cap like structure acrosome.

Human male ejaculates about 200-300 million sperms during a coitus. The seminal plasma along with the sperms constitutes the semen. The function of male sex secondary ducts and glands are maintained by androgen hormones.

Oogenesis:

The process of formation of mature female gametes is called oogenesis. It started during embryonic development stage when millions of ogonia (gamete mother cells) are formed in each fetal ovary.

The gametes mother cells start division and enter into prophase-I of meiotic division and get temporally arrested at that stage called primary oocytes.

Each primary oocyte get surrounded by a layer of granulosa cell than it is called the primary follicle.

At puberty, about 60,000- 80,000 primary follicles are left in each ovary.

Primary follicle gets surrounded by more layers of granulosa cells called secondary follicle that transform into tertiary follicle that contain fluid filled cavity called antrum.

The tertiary follicles further changes into the mature follicle called Graafian follicle, which rapture to release secondary oocytes (ovum) from the ovary by the process of ovulation.

Menstrual Cycle:

Menstrual Cycle: This is the reproductive cycle that starts from one menstruation till the next one. It mainly occurs in female primates like monkeys, apes, and human beings. The cycle repeats at an interval of 28-35 days and normally releases one egg per cycle. This cycle is important for the production of oocytes and for the preparation of the uterus for pregnancy. 

Menstruation: In this process, the blood and mucosal tissue are regularly discharged in a periodic manner. It occurs due to the breakage of the inner lining of the uterus. This process takes place once a month and is called a period.

Menarche: Menarche is the first menstruation for a human female that begins at puberty. The actual age for menarche generally differs from person to person. The first menstruation is the signal of the beginning of reproductive age in females.

Menopause: Menopause is defined as the permanent ceasing of the menstrual cycle in females. It occurs due to the depletion of oocytes and the loss of the ability of the ovary to produce estrogen as a result of aging. menopause. The average age of menopause is between 45-50 years, and it varies from person to person.

Menstrual Phase:

• In a 28 days menstrual cycle, the menses takes place on cycle days 3-5.
• The production of LH from the anterior lobe of the pituitary gland is reduced.
• The withdrawal of this hormone causes degeneration of the corpus luteum and, therefore progesterone production is reduced.
• Production of estrogen is also reduced in this phase.
• The endometrium of uterus breaks down & menstruation begins.
• The cells of endometrium secretions, blood & unfertilized ovum constitutes the menstrual flow.

Follicular Phase:

• This phase usually includes cycle days 6-13 or 14 in a 28 days cycle.
• The follicle stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete estrogens.
• Estrogen stimulates the proliferation of the endometrium of the uterine wall.
• The endometrium becomes thicker by rapid cell multiplication and this is accompanied by an increase in uterine glands & blood vessels.

Ovulatory Phase:

• Both LH & FSH attain a peak level in the middle of cycle (about 14th day).
• Estrogen concentration in blood increases.
• Rapid secretion of LH induces rupturing of graafian follicle and thereby the release of ovum.
• In fact, LH causes ovulation.

Luteal Phase:

• Includes cycle days 15 to 28.
• Corpus luteum secretes progesterone.
• Endometrium thickens.
• Uterine glands become secretory. 

Hormonal Control of Menstrual Cycle:

• FSH stimulates the ovarian follicles to produce estrogens.
• LH stimulates corpus luteum to secrete progesterone.
• Menstrual phase is caused by the increased production of estrogens.
• LH causes ovulation
• Proliferative phase is caused by the increased production of estrogens.
• Secretory phase is caused by increased production of progesterone.

Fertilization and Implantation:

The process of fusion of sperm with ovum is called fertilization.

During coitus (copulation) semen is released into vagina. The motile sperms swim rapidly to reach the junction of isthmus and ampulla of fallopian tube. The ovum also reaches there, and fusion of gametes takes place in at ampullary-isthmic junction.

In this acrosome of sperm undergoes acrosomal reaction and releases certain sperm lysins which dissolve the egg envelopes locally and make the path for the penetration of sperm.

These sperm lysins contain a lysing enzyme hyaluronidase which dissolves the hyaluronic acid polymers in the intercellular spaces which holds the granulosa cells of corona radiata together; corona penetrating enzyme (that dissolves the corona radiata) and acrosin (which dissolves the zona pellucida). Then it dissolves the zona pellucida.

Immediately after the entry of a sperm into the egg, the later shows a cortical reaction to check the entry of more sperms. In this reaction, the cortical granules present beneath the egg’s plasma membrane release chemical substance between the ooplasm and the plasma membrane (vitelline membrane). These substances raise the vitelline membrane above the egg surface. The elevated vitelline membrane is called fertilization membrane. The increased space between the ooplasm and the fertilization membrane and the chemical present in it effectively check the entry of other sperm. If polyspermy occurs, that is more than one sperm enter the secondary oocyte, the resulting cell has too much genetic material to develop normally.

The haploid gametes fuse together to form diploid zygote. As the zygote moves towards the uterus, the mitotic division starts and form cleavage to change into 2, 4, 8, 16 celled blastomeres.

The blastomeres with 8 to 16 cells are called morula. Morula divide to change into blastocysts. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The outer layer of blastocyst is called trophoblast that attach with endometrium of uterus, called implantation that leads to pregnancy.

Pregnancy and embryonic development:

The finger-like projections on trophoblast after implantation called is called chronic villi that along with uterine wall forms functional unit between developing embryo and maternal body called placenta. Placenta is attached with foetus with an umbilical cord that transport food and oxygen to embryo.

Hormones hCG (human chorionic gonadotropin), hPL (human placental lactogen) and relaxing are produced in woman only during pregnancy by placenta.

After implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm. These three layers give rise to all tissues (organs) in adults. It is important to note that the inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs

In human, after one month of pregnancy the embryo’s heart is formed. By the end of 2nd month limbs and digits are formed. By the end of 12 months, major organs and external genital organs are well developed. The first movement of foetus is observed in 5 months. By the end of 24 weeks body is covered with fine hair, eye lids and eyeless are formed. At the end of 9 months foetus is fully developed.

Parturition and Lactation:

Parturition-the process of delivery of fully developed foetus is called parturition. Signals for parturition originate from the fully developed foetus and placenta inducing mild uterine contractions called Foetal ejection reflex

It triggers the release of oxytocin from maternal pituitary The mammary glands of female, start producing milk, to the end of pregnancy by the process of lactation. The milk produced during the initial few days of lactation is called colostrum, which contain several antibodies.